Smallpox is a particularly dangerous biological weapon because it can be manufactured in large quantities, stored for an extended period of time, and delivered as an infectious aerosol. It is highly infectious and has a death rate as high as 25%. The only approved smallpox vaccine (live vaccinia virus) is available in very limited quantities (15.4 million doses) and is decades old. Current efforts are underway to produce enough vaccine to vaccinate the entire US population. A problem with this vaccination strategy is that a large segment of the population is susceptible to severe adverse reactions associated with the vaccine, including bloodborne dissemination of the vaccinia virus and even death. This threat is even greater now than when smallpox vaccinations were routine (prior to 1974) due to the growing population of immunosuppressed individuals. The broad long-term objective of the project is to develop bispecific antibodies (Heteropolymers, HPs) for treatment of complications associated with the administration of smallpox vaccine. In the present work, we hypothesize that vaccinia virus, the active component of smallpox vaccine, can be bound to erythrocytes (Es) via HPs, cleared to acceptor macrophages, and destroyed without killing the target cells. The HPs will consist of one monoclonal antibody (MAb) against a vaccinia protein expressed on the surface of extracellular (EEV) or intracellular (IMV) virus, cross-linked to a second MAb specific for E Complement Receptor Type I (CR1). The goal of the present work is to identify at least one anti-vaccinia HP that will be a candidate for use in future primate studies to establish efficacy against vaccinia complications following administration to immunocompromised animals. To achieve that goal, we will screen the anti-vaccinia MAb panel to identify the high affinity antibodies that bind EEV or IMV forms in solution phase and, preferably, neutralize the virus. The selected MAbs will be used to prepare HPs by cross-Iinking to a MAb specific for human E CR1. The ability of these HPs to bind EEV or IMV forms of vaccinia and transfer it to acceptor macrophages will be tested. We will also determine whether HP bound vaccinia is infectious. We will determine whether these HPs can prevent vaccinia virus spread and pathology in immunocompromised transgenic mice (expressing human CR1).